Cotton provides much of the high quality fiber for the textile industry. The modification of cotton fibers characteristics to better suit the requirements of the industry is a major effort in breeding by either classical methods or by genetically altering the genome of cotton plants.
About 90% of cotton grown worldwide is Gossypium hirsutum L., whereas Gossypium barbadense accounts for about 8%. As in most flowering plants, cotton genomes are thought to have incurred one or more polyploidization events and to have evolved by the joining of divergent genomes in a common nucleus. The cotton commerce is dominated by improved forms of two “AD” tetraploid species, Gossypium hirsutum L. and Gossypium barbadense L. Tetraploid cottons are thought to have formed about 1-2 million years ago, in the New World, by hybridization between a maternal Old World “A” genome taxon resembling Gossypium herbaceum and paternal New World “D” genome taxon resembling Gossypium raimondii or Gossypium gossypioides. Wild A genome diploid and AD tetraploid Gossypium taxa produce spinnable fibers. One A genome diploids species, Gossypium arboreum, remains intensively bred and cultivated in Asia. Its close relative and possible progenitor, the A genome diploid species G. herbaceum also produces spinnable fiber. Although the seeds of D genome diploids are pubescent, none produce spinnable fibers. No taxa from the other recognized diploid Gossypium genomes (B, C, E, F, and G) have been domesticated. Intense directional selection by humans has consistently produced AD tetraploid cottons that have superior yield and/or quality characteristics compared to the A genome diploid cultivars. Selective breeding of G. hirsutum (AADD) has emphasized maximum yield, whereas G. barbadense (AADD) is prized for its fibers of superior length, strength, and fineness (Jiang et al. 1998—Proc Natl Acad Sci USA. 1998 Apr. 14; 95(8): 4419-4424).
A cotton fiber is a single cell that initiates from the epidermis of the outer integument of the ovules, at or just prior to anthesis. Thereafter, the fibers elongate rapidly for about 3 weeks before they switch to intensive secondary cell wall cellulose synthesis. Fiber cells interconnect only to the underlying seed coat at their basal ends and influx of solute, water and other molecules occurs through either plasmodesmata or plasma membrane. Ruan et al. 2001 (Plant Cell 13: 47-63) demonstrated a transient closure of plasmodesmata during fiber elongation. Ruan et al. 2004 (Plant Physiology—Vol 136: pp. 4104-4113) compared the duration of plasmodesmata closure among different cotton genotypes differing in fiber length and found a positive correlation between the duration of the plasmodesmata closure and fiber length. Furthermore, microscopic evidence was presented showing callose deposition and degradation at the fiber base, correlating with the timing of plasmodesmata closure and reopening. Furthermore, expression of a β-1,3-endoglucanase gene (GhGluc1) in the fibers, allowing to degrade callose, correlated with the reopening of the plasmodesmata at the fiber base.
WO2005/017157 describes methods and means for modulating fiber length in fiber producing plants such as cotton by altering the fiber elongation phase as described in Ruan et al 2001. The fiber elongation phase may be increased or decreased by interfering with callose deposition in plasmodesmata at the base of the fiber cells.
Furthermore, it would be interesting for modification of fibers through genetic engineering, to possess promoters which are preferentially or specifically expressed in fibers cells only, and/or which are expressed only from a particular fiber development stage on.
WO2004/018620 relates to an isolated nucleic acid molecule encoding an endogenous cotton chitinase and its promoter, which are preferentially expressed in fibers during secondary wall deposition. The polypeptide encoded by the nucleic acid molecule, a DNA construct linking the isolated nucleic acid molecule with a promoter, the DNA construct incorporated in an expression system, a host cell, a plant, or a plant seed are also disclosed. The document also relates to a DNA construct linking the isolated promoter with a second DNA as well as expression systems, host cells, plants, or plant seeds containing the DNA construct. Methods of imparting resistance to insects and fungi, regulating the fiber cellulose content, and methods of expressing a gene preferentially in fibers during secondary wall deposition are also disclosed.
It would be useful to have alternative promoters that would drive gene expression preferentially and/or strongly in fibers throughout secondary wall deposition, i.e., strongly and continuously (e.g. at >50% of its maximal activity) e.g. from the initiation of secondary wall deposition to its termination or e.g. from maturation stage on. The initiation of secondary wall deposition is defined as the time when the dry weight/unit length of a cotton fiber begins to increase or when the dry weight/unit surface area of any cell begins to increase via synthesis of new wall material containing more than 40% (w/w) of cellulose. In the case of cotton fiber of G. hirsutum L., this is expected to occur between 14-17 DPA when cotton plants are grown under typical conditions in the greenhouse or the field (day temperature of 26-34° C., night temperature of 20-26° C., light intensity greater than or equal to 1000 einsteins/m2/s, with adequate water and mineral nutrition). The end of the secondary cell wall formation and start of the maturation phase is usually around 35DPA in the case of cotton fiber of G. hirsutum L.
Furthermore, it would be useful to have alternative promoters that would drive gene expression only or preferentially in fibers while excluding or minimizing expression in other tissues.
The inventions described hereinafter in the different embodiments, examples, figures and claims provide improved methods and means for modulating fiber length by decreasing or increasing callose deposition at the base of the fiber cell at a particular time point. The inventions described hereinafter also provide fiber-specific and/or fiber-preferential promoters and promoter regions.